1. Field of the Invention
The present invention relates generally to electrical circuit fault indicators and, more particularly, to an indicator apparatus for use in an electrical circuit including a fusible element which fuses in response to a fault current of a predetermined magnitude experienced by the circuit, as well as to a current interrupting apparatus adapted to be interposed in a multiple phase electrical circuit and operable to prevent single phasing of the circuit upon the occurrence of a fault in one of the phases.
2. Discussion of the Prior Art
It is known to provide a current-responsive, resettable, violence-free low fault range vacuum-type switch interrupter in series connection with a high fault range current limiting fuse (CLF) to form a circuit protecting device adapted for use in padmounted, oil-filled switch gear and the like. Such an apparatus is disclosed in U.S. Pat. No. 4,323,871, to Kamp et al., which disclosure is incorporated herein by this express reference.
In U.S. Pat. No. 4,323,871, the circuit protecting apparatus is disclosed as including a vacuum contact assembly having bottle or other means defining a substantially evacuated chamber, with a pair of normally closed electrical contacts within the chamber. At least one of the electrical contacts is shiftable for opening and closing of the contact pair to alternately interrupt and establish a current path therethrough. A mechanism is operably coupled to the contact assembly for opening the contact pair when a fault of predetermined magnitude is experienced by the interrupter-protected circuit. The mechanism includes a spring biased, over center toggle motive assembly operably coupled to a shiftable contact shaft, along with a latching element electrically interposed in the circuit and serving to hold the motive assembly in the releasable, biased, cocked position thereof. The latching element is disclosed as including a current-responsive, temperature sensitive bimetallic element which is constructed for heating when it experiences a fault current and, in response to and as a direct result of such heating, for changing its physical configuration for unlatching purposes.
In addition to fault-induced tripping of the switch interrupter of the apparatus disclosed in U.S. Pat. No. 4,323,871, structure is also provided in the patented apparatus for selective manual opening and closing of the vacuum contact pair. This structure includes an appropriate external operating handle, and the latter can also be employed to reset the vacuum contacts when the latter have been opened either manually or in response to a fault current. Further, the vacuum switch interrupter is disclosed as being designed for series connection with a current limiting fuse. Such a CLF is of conventional construction and includes a housing and a silver fusible element within the housing which is designed to fuse in response to a relatively high magnitude fault current. A suitable CLF is disclosed in U.S. Pat. No. 3,863,187, which patent is incorporated into the present disclosure by this express reference.
In three phase circuits wherein respective vacuum interrupters in accordance with U.S. Pat. No. 4,323,871 are interposed in the separate phase lines, a mechanical connection in the form of elongated rods between separate switch interrupters and operably coupled to the motive assemblies thereof is disclosed for gang operation of the switch interrupters. Thus, when one phase conductor experiences a fault sufficient to actuate the associated switch interrupter, the remaining switch interrupters are likewise actuated. This prevents a situation where one or more of the phase conductors remain energized in the event of a fault in another phase conductor.
By providing the disclosed construction, it was an object of the patented apparatus to cooperatively design the CLF and vacuum switch interrupter such that the switch interrupter would operate upon experiencing the let through current resulting from operation of the current limiting fuse. In this manner the switch interrupter was arranged to always operate in response to operation of the current limiting fuse.
However, one problem found to exist in the known apparatus discussed above is raised by the variation in the operating characteristics of the current-responsive, temperature sensitive bimetallic element which occur during changes in certain ambient conditions. Specifically, it has been found that even when a vacuum contact assembly and a current limiting fuse are properly coordinated for operation within a given range of current and voltage conditions, the current required to cause a change in the physical configuration of the bimetallic element varies with changes in the ambient temperature of the insulating oil surrounding the bimetallic element.